Ceramics are a general class of compounds that are the product of treating earthy raw materials with heat. Many ceramics comprise silicon and its oxides. Some of the more common ceramics are clay products, such as brick, porcelain, glass, and alumina. Ceramics are known for their heat-resistance, hardness, and strength. Metals, which are easily machined, do not retain their machined form at high temperatures. Ceramics, however, retain their shape at extremely high temperatures, but are brittle and very difficult to machine into a desired shape. Materials engineers have directed a great deal of effort into finding compositions that are easily machined into a desired shape and are stable at extremely high temperatures.
Ternary ceramic compounds such as titanium silicon carbide (Ti.sub.3 SiC.sub.2), and related "3-1-2" phase ceramics, as well as the "H-phase" ceramics have been studied and identified as meeting these requirements; that is, they are easily machineable and heat-resistant. For these reasons ternary ceramic compounds have been used to construct workpieces of varied shapes having heat-resistant properties and high strength. International Patent Application WO98/22244, published on May 28, 1998, of Barsoum et al. for "Process for Making a Dense Ceramic Workpiece" describes a process for making workpieces from these types of ceramic compounds and is herein incorporated by reference.
The application of corrosion resistant coatings to different articles in order to protect their surfaces from degradation by oxidation or chemical attack is a vastly important field of study. Much effort has been devoted to extending the useful lives of articles subject to corrosion by coating the article with a corrosion resistant composition. Coatings are also applied to substrates for protection against wear. Coatings with corrosion-resistant and wear-resistant properties are applied in many different ways. Some are applied by dipping or painting, others are applied by chemical adsorption, and still others are applied by chemical reaction. Many coatings used to provide protection to surfaces are applied by thermal spraying processes.
Thermal spray processes are a well known family of coating technologies that include detonation guns, high-velocity oxyfuel spray processes, wire-arc spraying, and both air and vacuum plasma spraying. U.S. Pat. No. 5,451,470 of Ashary et al.; U.S. Pat. No. 5,384,164 of Browning; U.S. Pat. No. 5,271,965 of Browning; U.S. Pat. No. 5,223,332 of Quets; U.S. Pat. No. 5,207,382 of Simm et al.; and U.S. Pat. No. 4,694,990 of Karlsson et al., collectively describe thermal spray processes and are herein incorporated by reference.
The types of coatings applied by these thermal spray techniques have generally been grouped into two broad categories, carbides and non-carbides. The carbides applied by thermal spray processes are generally transition-metal carbides such as tungsten carbide, chromium carbide, and cobalt-based carbides. The non-carbides applied by thermal spraying processes include iron-nickel based alloys, copper-nickel-indium alloys, metals and alloys such as aluminum, zinc, steel, bronze, and nickel, and aluminum-polyesters. Some ceramics, such as alumina and titania, which offer good wear-resistance, can be applied as coatings using the extremely high temperature (usually greater than 11,000.degree. C.) plasma spraying technique. Yttria-stabilized zirconia (YSZ), another ceramic, is well known as a thermal barrier coating in applications subject to extremely high temperatures.
High-velocity oxyfuel spray processes are advantageous in that they provide excellent dense, adherent coatings. Also the equipment used is more portable than other thermal spray equipment. Unfortunately, the ternary ceramic compounds described above have dissociation temperatures in the general range of from about 1000.degree. C. to about 1800.degree. C., and most thermal spray processes, including high-velocity oxyfuel, have gas jet temperatures in excess of 2500.degree. C.